1. Technical Field
The present invention relates to a cracking furnace and more particularly to a tubular furnace for thermal cracking of an organic feedstock such as petroleum hydrocarbons.
2. Background of the Art
Cracking furnaces for the pyrolysis heating of petroleum hydrocarbons to produce olefins are known in the art. Typical petroleum feedstocks include, e.g., ethane, propane, and naphtha. Typical products include ethylene, propylene, butadiene, and other hydrocarbons.
FIG. 1A illustrates a typical cracking furnace arrangement. Cracking furnace 10 includes a heating section 11 and a convection section 12 which is offset from the heating section 11 for the reasons stated below. Burners 13 are positioned on the floor of the radiant chamber 18 of the heating section.
One or more tubular coils 14 are positioned in the heating section 11. The feedstock flows through tubes 14a of the coils and undergoes pyrolysis at the cracking temperature (usually 950° C. to 1200° C.) wherein saturated hydrocarbons are cracked to produce olefins and hydrogen. The flow rate of the feedstock through the tubes is adjusted to provide a desired residence time at the reaction temperature. After the cracking has proceeded to the desired degree, it is important to quench the gas flow emerging from the radiant chamber to halt the reaction since continued reaction might produce unwanted by-products. Gas flow exiting the radiant chamber 18 is passed through heat exchangers 15 to quench the reaction. These heat exchangers are usually positioned on top of the radiant chamber 18, thereby requiring the convection section 12 to be offset. The heating section 11 typically has a length L of about 20 meters, a width W of about 3.5 meters and a height H of about 13.5 meters. The tubular coils 14 are generally arranged in a plane which is parallel to the plane defined by the vertical and lengthwise axes of the convection section 12. The convection section 12 is generally a stack for exhausting the furnace flue gas to the atmosphere. Convection section 12 usually contains one or more sections 16 for heat recovery wherein the feed is preheated by the flue gas, as well as sections for stack gas treatment to reduce emissions of pollutants such as nitrogen oxides and sulfur oxides.
Recent trends in ethylene production plants have led to larger and more intensely fired cracking furnaces. The capacity of a typical heater have increased from 100,000 metric tons per year to 180,000 metric tons per year. It is desired to increase capacity to at least 250,000 metric tons per year. To accomplish the increased furnace capacity the coil length can be increased, thereby increasing the height of the radiant chamber. Or, the number of coils can be increased, thereby increasing the length of the radiant chamber. However, neither of these changes are desirable. If the height of the radiant chamber is increased, it becomes more difficult to heat the coils evenly. The convection section tube length limits the length of the radiant chamber. If the radiant chamber becomes much longer, then the convection section problems arise with the flue gas flow from the radiant section into the convection section.
EP 0,519,230 discloses a pyrolysis heater in which the vertical tubes of the tubular coils provided in a plurality of parallel rows with each row being in a plane perpendicular to a plane through the longitudinal axis of the convection section. That is, the coils are oriented at 90° from the conventional arrangement of coils as depicted in FIG. 1A. While this arrangement can provide significant advantages with respect to increasing furnace capacity improvements can yet be made in furnace construction to facilitate such an arrangement.
In a relatively wide furnace such as that described in EP 0,519,230, wherein the tube coils are perpendicular to the longitudinal axis of the furnace, the flue gas can undergo recirculation within the radiant chamber. Referring now to FIG. 1B, a furnace 50 is shown with heating section 51, convection section 52 and burners 54. Flue gas flows are illustrated by arrows A, B, and C. While flue gas flows A and B tend to flow directly to the inlet opening 53 leading to the convection section 52, eddies C of flue gas can form, especially at the side of the chamber furthest away from the inlet 53 to the convection section where dead space tends to develop. These eddies result in inconsistencies in heating. Uniform heating throughout the radiant chamber is important for producing a consistent product and for facilitating process control.